This invention relates to a method of and apparatus for producing valve rotors.
Power assisted rack-and-pinion steering gear for motor vehicles generally comprises a rotary valve for directing fluid under pressure to double acting piston and cylinder means associated with the rack so that the fluid under pressure will act on the piston of said piston and cylinder means to assist the pinion in moving the rack axially in either of its axial directions of movement and so provide power assistance in the steering of the vehicle to which the steering gear is fitted. Such rotary valves generally comprise a rotor member of generally cylindrical configuration rotatable within a valve sleeve, the rotor member being connected or connectable with a steering column so that rotary motion will be imparted to the rotor member when a steering control connected to the steering column is rotated and the rotor member and the valve sleeve each having axially extending grooves therein which cooperate to control the flow of pressure fluid through the valve according to the angular disposition of the rotor member relative to the valve sleeve, the rotor member being connected with the pinion of the rack-and-pinion steering gear by way of a torsion bar so that rotary movement of the rotor member will be transmitted to the pinion by way of the torsion bar. The torsion bar serves to bias the rotor member towards an "on-centre" position. Generally, the rotor member will have an even number, e.g., six or eight, of said axially extending grooves, whereby the grooves can be divided into coacting pairs. One groove of each pair is connected by suitable passage means with an inlet port connectable to a source of fluid under pressure and the other groove of each pair is connected by suitable passage means with an outlet port connectable to a fluid reservoir. The valve sleeve likewise generally has an equal number of said axially extending grooves pairs formed therein with one groove of each pair being connected by suitable passage means to a first cylinder port connectable to one side of the double acting piston and cylinder means of power assisted rack-and-pinion steering gear and the other axial groove of each pair being connected by suitable passage means to a second cylinder port connectable to the other side of the double acting piston and cylinder means. The axially extending grooves in the valve sleeve are offset from those in the rotor member when the valve is in on-centre position, the grooves in the valve sleeve slightly overlapping those in the rotor member so that in the on-centre position of the valve fluid under pressure entering the groove in the rotor member connected to the inlet port can pass by way of one of the grooves in the valve sleeve into the groove in the rotor member connected to the outlet port and so pass to the fluid reservoir. As the rotor member is turned in one direction or the other from its on-centre position, so communication between the or each pair of grooves in the rotor member is gradually cut-off and communication between the inlet port and one of the cylinder ports and between the outlet port and the other of the cylinder ports is gradually increased so that one side of the double acting piston-and-cylinder means is connected to the source of fluid under pressure whilst the other side thereof is connected to the fluid reservoir. The longitudinal outer edges of the axially extending grooves in the rotor member are generally shaped to provide a required metered flow of the fluid under pressure through the valve to give required power-assist characteristics.
The shape or form which has to be imparted to the longitudinal outer edges of the axially extending grooves in the rotor member in order to obtain satisfactory power-assist characteristics is a complex curve, which may or may not have an abrupt recess or `window` therein, such as is described for example in British Patent Specification No. 1,308,992. The only satisfactory known method of imparting the required form to the longitudinal outer edges of the rotor grooves has been to introduce a coining or pressing tool having the required form thereon radially into each rotor groove so as to displace metal from the opposed longitudinal outer edges of the rotor groove radially inwards into the groove. As can be seen, metering takes place not across the opposed longitudinal edges of each rotor groove but across the adjacent longitudinal edges of adjacent rotor grooves. Since in the known method the adjacent longitudinal outer edges of adjacent rotor grooves are contoured independently of one another, inaccuracies can occur in the spacing between said adjacent longitudinal edges of adjacent rotor grooves and these inaccuracies can have a deleterious effect on the performance of the rotary valves. Moreover, the metal which is displaced radially into the rotor grooves can cause turbulences in the fluid under pressure flowing through the valve and so can disadvantageously affect fluid flow through the valve and/or increase noise levels in the valve.